A common requirement for an electronic circuit and particularly for electronic circuits including analog circuits that are manufactured as integrated circuits in semiconductor processes is a constant reference current. FIG. 1 depicts a constant Gm circuit of the prior art for providing a constant current Iref. A constant Gm circuit has a constant transconductance so the output current is ideally maintained at a predetermined current. If the circuit operated as an ideal circuit, current Iref would remain constant across variations in voltage supply Vdd variations and also be independent of process and temperature variations.
In FIG. 1, resistor R is implemented in the semiconductor process as an OD resistor and sometimes, a polysilicon resistor or combinations of these resistors. Transistors MP1, MN1, MN2, and MP2 provide a current mirror circuit wherein the current flowing through resistor R is also the reference current Iref at the circuit output. By selecting values for R and the sizes of transistors MP1, MN1, and matching the transistors MP2 and MN2 to MP1 and MN1 (note that as a known alternative, transistor size scaling can be used to vary Iref without changing the value of R), a predetermined reference current can be created at the output.
The current Iref is described by the expression:
  Iref  =            2                        μ          P                ⁢                              C            OX                    ⁡                      (                          W              L                        )                          *                  R          2                      ⁢                  (                  1          -                      1                          2                                      )            2      
Ideally, the reference current Iref would be independent of the temperature of the integrated circuit. In actuality, however, the terms R and the mobility term μPCox (W/L) in the denominator have temperature dependencies. Because the temperature dependence of the physical resistor R is not balanced with the temperature dependence of the mobility term, the current Iref that is observed in an actual circuit also has a temperature dependency. This is undesirable.
FIG. 2 depicts in FIGS. 2a, 2b and 2c the temperature dependency for an ideal and an actual mobility term, an ideal resistor and an actual resistor, and the resulting Iref current plotted over the usual temperature range for integrated circuits, −40 degrees C. to 125 degrees C., for the two cases. Because the mobility term (even in the ideal case) has negative temperature dependence, Iref also tends to have a temperature dependence that is significant, as the positive temperature dependence of the resistor R is not sufficient to compensate for it. Note the temperature dependence of Iref is positive (increases with increasing temperature), as it is proportional to the inverted mobility and resistor values.
As semiconductor processes advance, device sizes continue to decrease. Present semiconductor production includes 45 nanometer and soon 32 nanometer minimum feature sizes; these process milestones are usually referred to as “technology nodes”. Advances towards 28 nanometer node mass production are underway and expected shortly. The trend to smaller devices and more advanced nodes will continue.
As the device sizes shrink commensurate with the advances in the semiconductor technology nodes, the device characteristics and performance become dominated by physical layout effects. The devices also exhibit wider performance differences due to semiconductor process variations and temperature. For advanced semiconductor processes and future semiconductor processes, the temperature dependence shown in FIG. 2 may become even more pronounced.
Note in FIG. 2a that the ideal case, with the resistor a horizontal line indicating no temperature dependence, is not the optimum solution for a temperature independent Iref. This can be seen clearly by noting that in FIG. 2c, the lower curve in Iref at −40 degrees is the ideal case, and it ends up higher at 125 degrees C., because the mobility term μpCox(W/L)P in FIG. 2a has a temperature dependence, whether ideal or in an actual implementation. What is needed is a method to compensate the temperature dependence of the mobility term so that the Iref current is temperature independent.
FIG. 3 depicts in cross section two prior methods for forming the resistor R in a typical semiconductor process. FIG. 3a depicts an oxide diffusion resistor (OD resistor) 31 formed over the active area of the device between two conductors or metal lines 37, 39 that form the resistor terminals on a substrate 33. FIG. 3b depicts a polysilicon resistor 32 formed over the active area of a semiconductor substrate 33 between two conductors or metal lines 35, 37 that form the terminals of the resistor 41. These two approaches are sometimes used in combination to increase the resistance R. Nonetheless, additional improvements are still needed.
Thus, there is a continuing need for a constant Gm circuit that provides a temperature independent constant current source, while remaining compatible with existing and future semiconductor processes for integrated circuits.